The present invention relates generally to the field of atomic frequency standards and is more particularly directed to atomic frequency standards in which the energy state of an atomic ensemble is prepared by optical pumping using the coherence property of a laser light source.
In some prior art atomic frequency standards using alkali metal atoms, such as cesium 133 or rubidium 85 or 87 as the resonance source atoms, the energy states of the source atoms are prepared by optical pumping of a cell containing an admixture of alkali metal atoms and one or more buffer gases and using the intensity of a light source, particularly a laser light source, to achieve the optical pumping. More recently, atomic frequency standards have been developed in which the coherence property of a laser light source, rather than its intensity, is utilized to prepare the energy states of the source atoms and in which the quantum physics phenomenon of Coherent Population Trapping (hereinafter CPT) takes place wherein the ground state energy populations of the atomic ensemble remain unaltered. FIG. 1 hereof depicts a generalized scheme in which the CPT frequency standard is achieved. Referring to FIG. 1, there is provided a sealed, optically transparent resonance cell containing the alkali metal source atoms and buffer gases. The beam of a laser light source of appropriate wavelength for the particular alkali metal utilized as the source material is directed, seriatim, through a linear polarizer and a circular polarizer (.lambda./4 plate) and thence into said cell. Where a single laser light source is employed it is modulated over a frequency range, including a subharmonic of the hyperfine ground state 0-0 transition frequency of the alkali metal atoms contained in the resonance cell, thereby causing the laser to emit as sidebands two radiation fields whose frequency difference is equal to the hyperfine frequency of the alkali metal atoms contained in the resonance cell. Where two laser sources are employed, said lasers are phase-locked to one another with a frequency separation equal to the hyperfine frequency of the alkali metal atoms, thereby to also establish. two radiation fields of the type described above in respect of a single laser operated with two sidebands. The alkali metal atoms within the resonance cell are thus submitted to these radiation fields and resonance of said atoms takes place wherein a strong coherence of the ground state occurs at the hyperfine frequency and wherein transitions to the excited P state are inhibited. Thus, at resonance, all alkali metal atoms within the resonance cell are trapped in the ground state, no transitions take place from the ground state to the excited P state and no energy is absorbed from the laser radiation due to such transitions. The resonance phenomenon is signalled by: (a) a sharp increase in the intensity of the laser radiation transmitted through the cell along the laser beam axis and/or (b) by a sharp decrease in the intensity of fluorescence transmitted from the cell normal to the laser beam axis. Thus, either or both of these CPT resonance cell phenomena are detectable by stationing photodetector means: (i) to receive and detect the intensity of the laser light source beam transmitted through. the resonance cell along the beam axis and/or (ii) to receive and detect the fluorescent light generated within the resonance cell normal to the beam axis.
Frequency standards based upon the general CPT technology outlined above hold much promise in permitting substantial reduction in the size of atomic frequency standards from those of the prior art, due in large measure, to the absence, in CPT based standards, of the need for a relatively bulky and often operationally troublesome microwave cavity to surround the resonance cell. Both CPT based and other atomic frequency standards of current production have sizes averaging in the vicinity of 3.times.3.times.6 inches. However, it is obvious from even peripheral knowledge of, for instance, the fields of telecommunications, satellite navigation transmitters and receivers and the like, that further substantial diminution in size, and weight, of atomic frequency standards, already utilized extensively in these fields for extremely accurate and necessary timing functions, represents an important desideratum. In accordance with the present invention, such further diminution in size and/or weight from prior art atomic frequency standards is now permitted.
It is a principal object of the invention to provide a novel atomic frequency standard based upon the quantum atomic physics phenomenon of Coherent Population Trapping.
It is another object of the invention to provide an atomic frequency standard of the aforesaid type and of substantially diminished physical size occupying, say, a total volume of no greater than about 200 cubic centimeters.
It is another object of the invention to provide an atomic frequency standard of the aforesaid type which can be assembled with relative facility and rapidity.
It is still another object of the invention to provide an atomic frequency standard in which the elements of the optical physics package forming part thereof can be assembled relatively rapidly and in optically correct relationship.
Other objects and advantages of the present invention will, in part, be obvious and will, in part, appear hereinafter.